Lumens to Watts Calculator
Convert light output to power using efficacy.
LED ≈ 80–100, incandescent ≈ 15
Results update as you type.
How it works
Lumens and watts measure different things, light output versus power drawn, and this calculator relates them using luminous efficacy. You enter the lumens and the efficacy in lumens per watt, and it returns the wattage the light source consumes. Lumens quantify how much visible light a source emits, while watts quantify the electrical power it uses; the bridge between them is efficacy, the lumens produced per watt, which varies dramatically by technology. Traditional incandescent bulbs are inefficient at roughly 15 lumens per watt, halogen a bit better, compact fluorescents around 60, and modern LEDs highly efficient at 80 to 100 or more lumens per watt. This is why an LED producing the same 800 lumens as a 60-watt incandescent draws only about 8 to 10 watts. Dividing lumens by efficacy gives the power, so higher efficacy means lower wattage for the same brightness. This calculator is useful for comparing bulbs by their true light output rather than the outdated habit of shopping by wattage, estimating the power and running cost of a lighting setup, and choosing efficient lighting. Because manufacturers now list lumens on packaging, focusing on lumens for brightness and efficacy for efficiency is the modern approach. Set the efficacy to match your bulb type for an accurate result. Enter the lumens and efficacy to find the wattage.
Watts = lumens / luminous efficacy (lm/W)
Lumens measure light output, while watts measure power consumed. The relationship depends on luminous efficacy, how many lumens a source produces per watt. Dividing the lumens by the efficacy gives the wattage. Efficient LEDs produce far more lumens per watt than incandescent bulbs.
Source:Lighting / luminous efficacy
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter light output.
- 2
Adjust the options
Adjust luminous efficacy if they apply — sensible defaults are already set.
- 3
Read your result
Power updates instantly as you type — no button to press.
Frequently asked questions
How do I convert lumens to watts?
Divide the lumens by the luminous efficacy in lumens per watt. For example, 800 lumens at 80 lumens per watt is 10 watts. Because the result depends on efficacy, which varies by bulb technology, set the efficacy to match your light source for an accurate wattage.
What is luminous efficacy?
Luminous efficacy is how many lumens of light a source produces per watt of power consumed, measured in lumens per watt. It is the key to converting between lumens and watts. Higher efficacy means more light for less power. LEDs have high efficacy, while incandescent bulbs have low efficacy, wasting most energy as heat.
Why do LEDs use so much less power for the same brightness?
LEDs have far higher luminous efficacy than incandescent bulbs, producing 80 to 100 or more lumens per watt versus about 15 for incandescents. They convert much more of the electricity into light rather than heat. So an LED can match an incandescent's brightness at a fraction of the wattage, which is why they dramatically cut lighting energy use.
Should I shop for bulbs by lumens or watts?
Shop by lumens for brightness. Watts measure power, not light, and only indicated brightness in the era when nearly all bulbs were incandescent. With efficient LEDs, the same brightness comes at far lower wattage, so lumens are the correct measure of how bright a bulb is, while wattage now indicates energy use.
What efficacy should I use for my bulb?
Use roughly 15 lumens per watt for incandescent, about 20 for halogen, around 60 for compact fluorescent (CFL), and 80 to 100 or more for LED. Check the bulb's packaging, which often lists both lumens and watts, from which you can compute the actual efficacy. Setting the right efficacy gives an accurate conversion for your specific bulb.